Re: [patch 0/4] V2 ia64 SPARSEMEM

[Jack Steiner <steiner@sgi.com>]

On Wed, May 25, 2005 at 11:13:47AM -0400, Bob Picco wrote:
...
> and HPSIM.  An early version of ia64 with SPARSEMEM was tested by Jesse. It
> would be optimal to have another test pass on SGI NUMA hardware.


I'm away from the office this week, but next week, I'll give the patch
another spin on a big SGI system.

>
>
> VIRTUAL_MEM_MAP was introduced on ia64 because of memory holes on ia64
> platforms. The mem_map of the pglist_data is a pointer to a virtual
> contiguous array of page structures in memory for a node.  To
> eliminate memory waste contiguous memory holes don't have page structures.
> The alternative would be for holes in memory to be represented by reserve
> page structures.  The VIRTUAL_MEM_MAP solutions requires ia64_pfn_valid
> and a hook in the buddy allocator to check for holes in memory.
>
> SPARSEMEM has eliminated mem_map, ia64_pfn_valid and the buddy allocator
> hook.  There is a small cost for SPARSEMEM.  Any section which has both
> memory and holes requires reserved pages for the holes.  I can see

Our systems are notorious for having large holes in memory on nodes. For example,
a node may have memory at node offsets 0, 16GB, 32GB & 48GB. At each offset, you
can have a chunk of 1 to 16GB. For example, a typical system might have memory 
at the following node offsets:
    Memory at
         0 -  4GB
	16 - 20GB
	32 - 36GB
	48 - 52GB

You are guaranteed to have memory at node offset 0. Memory may or
may not exist at the other offsets.

I haven't had a chance to look at the patch, but will page struct entries 
will be allocated for the pages in the holes (4GB-16GB, etc).



-- 
Thanks

Jack Steiner (steiner@sgi.com)          651-683-5302
Principal Engineer                      SGI - Silicon Graphics, Inc.